1. Field of the Invention:
The present invention relates to a method of and an apparatus for manufacturing a tube having holes, more particularly, which is suitable for use in irrigation or sprinkling.
2. Description of the Related Art:
Conventional tubes having holes have a large number of small holes formed in its longitudinal direction on a half side thereof. Such tubes having holes are widely used for sprinkling water or spraying liquid fertilizer over vegetables, fruits or flowers.
Manufacture of such a tube is not very difficult; it can be easily manufactured by forcing a synthetic resin through the cylindrical orifice of a die. However, drilling small holes only on one side of the continuous tube manufactured is not easy.
Conventionally, a tube having holes is manufactured by preparing a first synthetic resin tape with a large number of holes drilled in it and a second synthetic resin tape having the same shape but no holes in it and then by sticking the edges of the first tape to the edges of the second tape to form a continuous tube. The first tape is drilled mechanically by punching or using a laser beam.
However, the tube having holes manufactured in the above-described method has a disadvantage in that it may rupture at its seams when a hydraulic pressure is applied thereto. Furthermore, it is not easy to mass produce the above-described type of tube.
Accordingly, it has been proposed to drill a seamless tube manufactured by the extrusion by folding the seamless tube into two and then notching the folded edge of the tube in a semicircular form at predetermined intervals. The semicircular notches formed become circular holes when the seamless tube is unfolded. However, it is very difficult to notch the edge of the folded tube in the same semicircular form, and this makes it difficult for substantially circular holes to be formed in an unfolded tube, causing the areas of the openings of the individual holes to vary from each other.
When drilling is conducted using a laser beam, as stated above, the laser beam is condensed by a lens so as to achieve a high energy level required for local processing, and the condensed laser beam is focused on the surface of a tube. The depth of a hole that can be formed by the laser beam is about .+-.2 mm.
It is therefore required that the relative distance between the tube and a laser beam irradiating portion be maintained precisely at a value less than the above-described depth, and preferably, at .+-.0.5 mm or less. However, no adequate method of maintaining this precision is known, and drilling failure often occurs.